Hey Moe wooohooo nyuk nyuk
That's not a virus. The pop-up comes from unchecked ads.
Without looking at your computer, I'd guess its a fake anti-virus or fake free software download.
When you see something like that, fear not. The only solution at that point it not to click cancel but to kill the browser itself. No need to pull the plug on your computer or hit the power switch.
To kill the browser open up taskmgr.exe the task manager either from the start button or [CTRL] + [ALT] + [DEL] and select task manager.
Look for iexplorer.exe or firefox.exe and end it.
You will lose everything you didn't save.
If you're running Linux, you should already know what to do with /bin/kill.
nyuk nyuk pop whhoooowhhhoooo